|  | 8.2 | Network Security
| 8.2.3 | Back up operations
The five types of back up operations are as follows: Full backup: All files on the disk are stored to tape and the archive bit for all files is set to off. Incremental backup: Backs up all the files that have been created or modified since the last full backup. It is important to remember two things about an incremental backup. One, that it only works in conjunction with a full backup and two, that any file created or modified has its archive bit turned back on so it will be saved to tape during the next incremental backup. Differential backup: Backs up all the files that have been created or modified since the last full backup. This sounds the same as an incremental backup, but the difference is that even though the file is saved to tape, the archive bit is not reset. This means that each time a differential backup is done, all of the files modified or created since the last full backup will be stored again. Copy backup: Backs up user selected files to tape. This backup also does not reset the archive bit to off. Daily backup: Backs up only the files that are modified on the day of the backup. This backup also does not reset the archive bit to off. The first three backup procedures are the most widely used. Here is a sample way of doing first an incremental backup and then a differential backup. To do an incremental backup, first you would do a full backup on Monday; this would reset all of the archive bits on the files. On Tuesday an incremental backup would be performed to a separate tape. This will store all of the files modified on Tuesday to tape and reset their archive bit. This process is repeated for all the other business days of the week, each with a separate tape. This gives a complete backup of all files modified during that week. On the following Monday the entire process starts over again. The advantage of this type of backup scheme is that it requires the least amount of time per day to do the backup, so it has the least impact on the network resources. The disadvantage is that if you need to restore the backup it requires that you first restore the full backup tape and then all of the incremental backup tapes in order, which takes a great deal of time and if one the tapes is bad you lose that information. To do a differential backup, first a full backup would be done on Monday; this would reset all of the archive bits on the files. On Tuesday a differential backup would be performed to a separate tape. This will store all of the files modified on Tuesday to tape, but it won't reset their archive bit. This process is repeated for all the other business days of the week, each with the same tape. This process also gives a complete backup of the network data. Its advantages are that it only requires two tapes to make and restore the backup if necessary. The disadvantages of this method are that each day the files that were backed up on previous days are stored again which takes a lot more of the network resources per day. Also, if the differential backup tape is damaged and the restore was preformed on Friday, four days worth of data would be lost and have to be reentered. Just as a note, a full backup that is done each day only requires one tape to restore the data, but it is impractical to run a full backup each day due to the amount of dedicated time it requires. Neither the copy backup nor the daily backup reset the archive bit, and are used for backup of selected files. Another important consideration when doing system backup is the data that is on the user workstations. Is the data stored on a workstation being backed up and if so, how? The data that is stored on the workstations is just as, and sometimes more important than the data stored on the network servers. The particular method of backup for workstations depends on the situation. Here are some different scenarios for workstation backup. The first method would be used for a workstation that creates and works on a large amount of data, which is only used by that workstation. In this case an individual tape drive might work best. It will allow for very large amounts of data to be backed up and does not impact the network throughput. The downside of the method is that it puts the responsibility for the backup in the hands of the user. A second way of doing workstation backup is to copy all data files to a removable storage device such as a floppy or ZIP disk drive. This saves the expense, time, and complication of doing a tape backup, but still leaves the responsibility in the hands of the user. Finally the last method to be mentioned, would be to create directories on the servers for all users to store their data. This solution removes the user's responsibility of doing the backup. It is done when the servers are backed up and eliminates the need for special devices on the workstation to do the backup. The drawbacks of this solution are that the policies of where data is to be stored must be clearly defined. The users must understand where they are storing the data to make sure it is done correctly. Also if there is a network communication problem the data may not be available to the users until the problem is corrected. As you have seen, with any solution there are potential problems. Each situation will have a "best case" solution for that particular time and place. The only wrong solution is to ignore the need to backup all data on the system.
| Web Links
| The Importance of Backups
| Backup Methods, Devices, and Media
| Rotation Schedule for Backup Media